The present invention relates to wireless communication systems, and particularly relates to estimating communication channel error rates.
Error rate estimation serves many purposes in wireless communication systems. As one example, the transmit power control mechanisms widely employed in cellular communication networks based on Code Division Multiple Access (CDMA) use channel error rate as a control variable in their power adjustment algorithms. More particularly, CDMA-based radio base stations generally control the reverse link transmit powers of the mobile stations being supported by them based on transmitting Transmit Power Control (TPC) bits to the mobile stations. Similarly, each mobile station generally controls the forward link transmit power of the radio base station(s) transmitting to it on a dedicated forward link traffic channel by transmitting TPC bits to the radio base station(s).
The transmitted TPC bits generally take on one of two values: a logical “1” or “UP” command to indicate that the remote transmitter should increase its transmit power, and a logical “0” or “DOWN” command to indicate that the remote transmitter should decrease its transmit power. The value of each TPC bit is determined by comparing the received pilot signal with a signal strength target, usually expressed as a Signal-to-Noise Ratio. For a given measurement interval, the receiver compares the received pilot signal strength to the target and transmits a DOWN command if the received pilot signal strength is above the target and transmits an UP command if the received pilot signal strength is below the target. By making the comparison many times per second, the receiver generates a steady stream of TPC bits, thereby keeping the received pilot signal strength at the target. Generally, the transmitter transmitting the pilot signal also transmits one or more traffic channels at defined traffic-to-pilot power ratios, meaning that power control of the pilot signal operates as power control of the associated traffic channel(s).
Such received signal strength processing carries the label “inner loop” power control and, as the name suggests, inner loop power control generally is paired with “outer loop” power control. While the inner loop power control process ensures that the received pilot signal strength is kept at the target, the outer loop power control process ensures that the appropriate target is being used by the inner loop power control.
For example, outer loop power control commonly computes or predicts an error rate for the data being received in association with pilot signal reception and compares that to a defined upper error limit, e.g., 10%. Such error rates usually are expressed as Block Error Rates (BLERs) or Bit Error Rates (BERs). Regardless, if the error rate exceeds the defined upper limit, the outer loop power control adjusts the inner loop's target upward. Conversely, if the error rate falls below a lower limit, e.g., 1%, the outer loop power control adjusts the inner loop's target downward.
Implicit in the above description of outer loop power control is the availability of “coded” data for determining reception error rates—i.e., data that includes or is accompanied by error detection and/or error correction information. Without coded or known data, such as pilot bits, no explicit control variable exists for assessing reception error performance and thus no explicit basis exists for determining the up and down adjustments of the inner loop power control target. The Fractional Dedicated Physical Channel (F-DPCH) recently introduced in the Wideband CDMA (WCDMA) standards includes power control information (e.g., TPC bits), but does not include data with error correction information. Thus, the F-DPCH exemplifies the type of channel that complicates the traditional inner/outer loop power control approach.